Add support for `react-testing-library`

Changes

This version adds variants of the Wrapper classes, with some additional tools for react-testing-library.

To cater for one or the other, the import paths had to be changed, so now instead of this:

import { Wrapper } from "react-test-wrapper";

You now have to use one of these depending on which tooling you'd like to use:

// For enzyme
import { Wrapper } from "react-test-wrapper/enzyme";

// For react-testing-library
import { Wrapper } from "react-test-wrapper/react-testing-library";

For more details, please refer to the documentation in the README.